Electrical power generation systems powered by variable speed prime movers that require highly regulated electrical output, such as electrical power generation systems used for aeronautical applications, generally use a wound field synchronous machine (WFSM) that serves as an electrical generator. This is because it is easy to adjust rotor current to regulate electrical output of a WFSM. An electrical power generation system may alternatively use a regulated permanent magnet machine (PMM) that has a control coil. Current level in the control coil regulates output current of the PMM. A control coil current regulator system senses electrical output potential on a DC bus for the electrical power generation system with such a PMM and adjusts the current level in the control level to regulate the DC bus potential.
In any case, the electrical generator for an electrical power generation system requires a source of mechanical power, typically a prime mover that comprises a gas turbine engine, coupled to it so that it may convert the mechanical power into electrical power. Coupling is by way of a mechanical coupling that comprises at least a driveshaft, and more typically a reduction gearbox and driveshaft combination. For electrical power generation systems of a dual generator design, a requirement that a single failed generator that causes the shear of a driveshaft not propagate through the gearbox in such a manner as to result in a failure of the driveshaft for the other generator may constrain the mechanical torsional characteristics of the mechanical coupling. The inclusion of relatively soft generator input driveshafts may prevent such shaft shear propagation. However, because of this and other design constraints, the torsional resonant frequencies of the mechanical coupling can occur within the frequency range of an electrical potential regulation system for the electrical power generation system. Such mechanical coupling is subject to various torsional oscillations during operation of the electrical power generation system. During some operating conditions these torsional oscillations may persist, becoming sustained torsional oscillations (STOs). These STOs may lead to fatigue damage of the coupling and may cause catastrophic damage if undetected.
A co-pending patent application to Markunas et al., U.S. Ser. No. 11/824,659, filed 2 Jul. 2007 and having both common inventorship and ownership with this Application, describes an active damping system to detect and suppress STO by means of an STO signal detector that detects an STO signal in the generator output and a feedback loop that introduces a modulation of the generator control current that is out of phase with the detected STO signal to dampen the STO. This Application incorporates Markunas et al. by reference.
However, Markunas et al. does not address the function of actually indicating the presence of STO or measuring the torque magnitude of the STO, either as part of such a system to ascertain that the damping action functions properly or as a stand-alone STO monitoring system to measure STO and predict mean-time to failure (MTF) of the mechanical coupling arrangement based on the frequency and magnitude of the STO.